Many data storage media such as optical media or magnetic media have been developed to store data. With the increasing development of digitalized generation, the data storage density for the conventional data storage media is unsatisfactory. Nowadays, for dealing with such a problem, a holographic storage technology is developed by using holographic storage media to store data. A holographic storage medium has a largest capacity of about 3.9 TB (terabyte) among various storage devices. Perhaps, the holographic storage technology is succeeded after the HD-DVD or Blu-ray technology to be the most popular data storage technology.
In a typical holographic storage system, a laser beam is split into two beams by a beam splitter. These two beams are served as an object beam and a reference beam, respectively. The object beam and the reference beam are coherent. The object beam illuminates the object (pixel data or data page on data plane) and then the object scatters light onto the holographic storage medium. The object beam interferes with the reference beam to form an interference pattern such that a diffraction grating is recorded in the holographic storage medium. For reading pixel data from the holographic storage medium, the interference patterns recorded in the holographic storage medium are diffracted with the reference beam and thus a reconstructed object beam is obtained. Then, the reconstructed object beam is detected by a photo detector for obtaining the pixel data.
It is very important to remain optically stable in order for making and viewing a hologram. Any relative movement of the object beam and the reference beam may result in image shift when the pixel data are detected by the photo detector. Due to the image shift, the pixel data read from the holographic storage medium are inaccurate. For compensating the pixel data, a holographic storage system is disclosed in U.S. Patent Publication No. 2005/0286388, which is assigned to InPhase Technologies, Inc. and the contents of which are hereby incorporated by reference. In such a holographic storage system, predetermined reserved blocks are assigned throughout each data page. By searching the reserved blocks, the image shift is realized. According to the image shift, the position errors are determined. The pixel data are then compensated according to the corresponding position errors. Therefore, after a data page is received by the holographic storage system, it is very important to match a pattern of the data page to the predetermined reserved blocks. If the matching results are satisfied, pixel data may be accurately detected and further decoded.
U.S. Patent Publication No. 2005/0286388 also discloses a method of searching a reserved block. In accordance with InPhase's design, a reserved block having 8×8 pixel data of known pixel patterns is served as a reference reserved block. When an image file with 64×64 pixel data is read by the holographic storage system, a series of unit blocks with 8×8 pixel data are successively scanned and then correlations associated with respective unit blocks and the reference reserved block are calculated. The correlation between a specified unit block A and the reference reserved block B may be deduced by the following equation:
  Correlation  =                    ∑        m            ⁢                        ∑          n                ⁢                              (                                          A                mn                            -                              A                _                                      )                    ⁢                      (                                          B                mn                            -                              B                _                                      )                                                                    ∑            m                    ⁢                                    ∑              n                        ⁢                                          (                                                      A                    mn                                    -                                      A                    _                                                  )                            2                                      -                              ∑            m                    ⁢                                    ∑              n                        ⁢                                          (                                                      B                    mn                                    -                                      B                    _                                                  )                            2                                          
In the above equation, Amn denotes the brightness intensity of the pixel at the m-th row and the n-th column of the specified unit block A, Bmn denotes the brightness intensity of the pixel at the m-th row and the nth column of the reference reserved block B, Ā denotes the average pixels intensity of the unit block A, and B denotes the average pixels intensity of the reference reserved block B. Generally, the correlation between the unit block A and the reference reserved block B is ranged from 0 to 1. Ideally, the unit block A having correlation of 1 with respect to the reference reserved block B is the reserved block.
In the real application of the holographic storage system, however, the correlations for all unit blocks A with respect to the reference reserved block B are less than 1 because it is difficult to maintain optically stable. As a consequence, the unit block having the highest correlation with respect to the reference reserved block is usually deemed as the reserved block.
Since the computation associated with the correlation is very complicated, it is time-consuming to compute the correlations of all unit blocks with respect to the reference reserved block when an image file is received by the holographic storage system. In other words, since it takes much time to acquire a reserved block, the reading speed of the holographic storage system is very slow.
Therefore, there is a need of providing a method for quickly acquiring a reserved block in a holographic storage system so as to obviate the drawbacks encountered from the prior art.